Herbivorous (plant-eating) and omnivorous (plant- and meat-eating) fish include damselfish, gobies, parrotfish, rabbitfishes, Nile tilapia and bluefin tuna. Herbivorous and omnivorous fish can be found in both fresh and saltwater.

There are several species of reef fish that have been observed feeding on plant matter; these include the yellow-and-black moorish idol, which has a long, extended fin and may be recognizable to saltwater aquarium and snorkeling enthusiasts. The bluefin tuna, a large saltwater fish with a voracious appetite, has been observed consuming kelp, though it appears to prefer a carnivorous diet. Piranha are a freshwater fish with a notoriously large appetite; similar to the bluefin tuna, though these fish prefer meat, they have been observed eating plant matter.